1. Field of the Invention
The present invention relates to a method for load modulation in a combination comprised of a transmitting oscillator circuit and a receiving oscillator circuit, whereby a voltage on a transmitting oscillator circuit is modulated by retroaction of a voltage change in a receiving oscillator circuit.
Furthermore, the invention is directed to a circuit arrangement for load modulation in a receiving oscillator circuit, which can be mutually coupled with a transmitting oscillator circuit, having at least one inductance, one capacitance, and one controllable impedance.
2. Description of the Background Art
A method and circuit arrangement, particularly from RFID (Radio Frequency Identification) applications, have been known per se. In this document, every application, where a transmitting oscillator circuit supplies an inductively coupled receiving oscillator circuit with energy and, as the case may be, reads data via the receiving oscillator circuit, can be considered an RFID application. Combinations such as these are used, for example, for object identification, whereby via a receiving oscillator circuit, a transmitting oscillator circuit of a reader responds to an object marked with a so-called tag and retrieves information.
To establish contact, the transmitting oscillator circuit of the reader creates a high-frequency magnetic field, which induces an alternating voltage in an inductance of a receiving oscillator circuit, which is located in close proximity to a reader. The alternating voltage induced in the receiving oscillator circuit is demodulated and serves, for example, as energy supply for an integrated circuit connected to the receiving oscillator circuit. Furthermore, a clock frequency is derived from the induced alternating voltage, which is available to the integrated circuit, that is, for example, a microprocessor and/or a storage element for a system clock. By complementing the inductance of the transmitting oscillator circuit and/or the receiving oscillator circuit with capacitances, particularly with capacitances that are parallel to oscillator circuits, resonance effects are produced, which substantially improve the efficiency of the energy transfer.
The transmission of data from the reader to the receiving oscillator circuit (downlink) can be effected, for example, by activating and deactivating the magnetic field. For data transmission in a reverse direction from the receiving oscillator circuit to the reader, a load modulation can be used, which presupposes a sufficient proximity (distance less than 0.16* wavelength) of the transmitting and the receiving oscillator circuit. With sufficient proximity, a so-called mutual coupling occurs, whereby the energy absorption of the receiving coil is realized in voltage alterations in the transmitting oscillator circuit due to retroaction on the transmitting oscillator circuit. Controlled modulations of the load, namely, the impedance of the receiving oscillator circuit, thus create voltage alterations in the transmitting oscillator circuit, which can be evaluated for data transmission.
With the quality of the inductances used in the receiving oscillator circuit ever improving, in other words, with an increasing ratio between reactance and active resistance, the attenuation of the oscillator circuit and the width of the resonance curve decrease. In other words, the use of better-quality coils results in greater frequency selectivity and, with the voltage on the reader side unchanged, a higher voltage on the tag side, which increases the range of the communications connection.
In this context, it is known per se to reduce, or limit, the voltage of the receiving oscillator circuit to certain values (terminal voltages) by reversing or switching between two voltages within the framework of the modulation. This is done by inserting depletion layer elements between oscillator circuit terminals and a reference potential, or mass potential. For example, a lower terminal voltage is realized when the forward voltage above the depletion layer elements drops, whereby in a first approximation, the voltage drop is independent from the current due to the exponential dependence of the current from the voltage. In other words, unlike with ohmic resistance, the voltage drop is not linear with the current flow but remains approximately at the level of the forward voltage, even at higher current intensities.
As a result, the depletion layer elements act as a reliable restriction of the voltage of the oscillator circuit to a corresponding value, even at high coil voltage. This is of particular importance with systems that have high-quality inductances, which at close spatial proximity of the transmitting oscillator circuit and the receiving oscillator circuit could otherwise cause undesirably high voltages.
The second, upper terminal voltage can be realized with, for example, a Zener diode having a reversed forward direction and connected in series that can be controlled or connected to short circuit. In the short-circuit mode, the described restriction is on the lower terminal voltage, whereas in a non-short circuit mode, the breakdown voltage of the Zener diode provides an additional voltage offset, which in sum with the aforementioned forward voltages defines an upper terminal voltage. In a mode, when the Zener diode is short-circuited, a comparatively large flow emerges from the receiving oscillator circuit, which corresponds with the loaded mode of the oscillator circuit. The current drain from the oscillator circuit as well as the demand on the oscillator circuit from opening the short circuit via the Zener diode is decreased accordingly.
With these known load modulations, the following problem has been observed: When at the activation of the modulation a high coil current is being induced, that is, when the oscillator circuit voltage on the lower terminal voltage is restricted, the coil current could leak via the Zener-diode by-pass and into the remaining depletion layer elements that are switched in a forward direction, whereby the voltage of the oscillator circuit can fall below the terminal voltage and even below a threshold value that is used for the detection of oscillations (pulses) of the voltage of the oscillator circuit. That is to say, at unfavorable phase conditions, it can happen that when the load is turned on, the voltage of the transmitting oscillator circuit drops below a detection threshold for one or several periods due to retroaction, which falsifies the information transmission. This can result in a loss of data during information transmission to the reader.
If, during a high-induced coil current, the modulation is turned on, the depletion layer elements assure a restriction of the oscillating current voltage to a value that is predetermined by the depletion layer elements. In this phase, the diodes have the effect of a direct-current source and thus do not attenuate the coil current sufficiently so that the induced oscillation is altered. This results in a widening of the straight-fitting clock pulse phase (pulse widening), which leads to at least a partial cancellation of the sequential oscillation. This is manifested in that at least one oscillation in the amplitude is not large enough for a preset detection threshold.